In compression molding a raw material such as powder or granules into tablets of pharmaceuticals, quasi-drugs, cosmetics, agricultural chemicals, feeds, foods or the like, a mold comprising a combination of a mortar having through-holes corresponding to the shape of tablets with a lower pestle and an upper pestle to be inserted into the through-holes (mortar holes) has hitherto been used. In a tablet molding machine using the above mold, a raw material such as powder is filled into the mortar into which the lower pestle has been inserted, and the raw material is compressed by the upper pestle for molding into desired tablets.
As described, for example, in Japanese Patent Laid-Open No. 8540/1995, for example, iron-base alloys such as alloy tool steels, for example, SKS2 and SKD11, or cemented carbide alloys composed mainly of compounds of Mo (molybdenum), W (tungsten) and the like have hitherto been adopted in molds used, for example, in tablet molding machines.
Further, in order to improve corrosion resistance of molds such as alloy tool steels, an attempt has also been made to coat the surface with a chromium plating. However, satisfactory effect cannot be attained due to the separation of the plating layer. The chromium plating layer can have a given effect for an improvement, for example, in surface hardness. Since, however, the chromium plating layer per se is disadvantageously easily separated, satisfactory and stable wear resistance improvement effects and the like cannot be attained. This has led to a demand for an improvement, for example, in corrosion resistance and wear resistance while maintaining strength and hardness of the member for a mold.
In order to solve the problem of wear resistance, Japanese Patent Laid-Open No. 62595/2001 describes high hardness and high corrosion resistance tablet molding pestle and mortar. This alloy has high hardness and high corrosion resistance and, at the same time, has releasability. Although this alloy can maintain good releasability for approximately a few hours immediately after tablet molding, a further improvement in releasability has been desired for mass production purposes. Further, since this alloy has a relatively low fatigue strength, an increase in strength has been desired, and, in addition, the possession of a planishing property of the molding face has also been desired.
On the other hand, applications in which corrosion resistance is required include not only manufacturing equipment such as the above-described mold for corrosive powder but also processing equipment for chemicals, processing equipment for waste liquids or waste sludge, combustion apparatuses, and their peripheral components. Further, corrosion resistant steels such as stainless steels have been used in applications where corrosion resistance is mainly required, for example, molds for resin lenses or engineering plastics or other resins, and components such as cutting tools and direct acting bearings. Corrosion resistant steels such as stainless steels, however, are unsatisfactory, for example, in strength and hardness and, thus, cannot be used in applications where hardness and wear resistance are particularly required.
For example, Japanese Patent Laid-Open No. 18031/1988 describes a high corrosion resistance hot pressing mold comprising 20 to 50% by mass of Cr (chromium) and 1.5 to 9% by mass of Al (aluminum) with the balance consisting essentially of Ni (nickel). This hot pressing mold has such properties that it exhibits high hardness against hot pressing under conditions of temperature 500 to 800° C. and pressing pressure 500 to 2000 kg/cm2 (50 to 200 MPa) and has buckling resistance. Further, the mold has been found to have corrosion resistance against Ni and Cr. So far as the present inventors know, however, this mold component of an Ni—Cr—Al-base alloy possesses excellent material hardness and corrosion resistance, but on the other hand, the wear resistance is not always satisfactory and, for some service conditions, wear progresses in a sliding part of the component, disadvantageously leading to shortened component service life.
Good planishing properties are required of molds for resin lenses and resins such as the so-called “engineering plastics.” Since, however, the conventional steel product is an alloy which is hardened by a relatively large precipitated carbide, pores are formed due to falling of precipitated carbide particles during polishing and, in addition, damage to the polished surface by fallen particles, making it difficult to conduct planishing. Further, in the conventional steel material, Ni plating or CrN coating is carried out for releasability improvement purposes. The conventional steel material, however, is disadvantageous in that the releasability is not satisfactory, the releasability is deteriorated depending upon surface roughness, and the releasability varies depending upon wear.
In order to improve the wear resistance, Japanese Patent Laid-Open No. 88431/2002 describes a member comprising a case hardened layer provided on this Ni—Cr—Al-base alloy. A further improvement in releasability, an improvement in fatigue strength, and an improvement in planishing properties of the molding face have been desired. In particular, molds for resin molding had a serious problem involved in the production thereof associated with releasability that the molding resin is likely to adhere to the mold.
The realization of a homogeneous metal structure is desired for improving the releasability, the fatigue strength, and the planishing property of the molding face. That is, when an unaged structure is present, in molding powder or the like, the powder is cut into the unaged soft phase and the amount of the powder adhered is gradually increased, resulting in deteriorated releasability. Further, since the unaged soft layer is present, the fatigue strength is lowered. Furthermore, there is a tendency that a difference in hardness between the aging precipitated phase and the unaged phase affects polishing and causes a difference in polishing between the aging precipitated phase and the unaged phase, leading to a tendency that planishing becomes difficult. As reported in Materia Japan (a bulletin of The Japan Institute of Metals), Vol. 22, No. 4, p. 323, in the precipitated phase of this alloy system after aging treatment, a γ phase is composite precipitated in a thin layer form at boundaries between the layered α phase and γ matrix phase to form a characteristic three-layer structure of α, γ′, and γ parent phases. In this conventional production process of this alloy, even after aging heat treatment at a proper temperature of 650° C. to 800° C., a certain level of an unaged γ phase stays, and, thus, a complete three-phase (α, γ′, and γ) structure cannot be realized.
Accordingly, in order to improve the releasability, fatigue strength, and the planishing property of the molding face, a reduction in unaged phase and homogeneous refinement have been desired. Further, stable precipitation of three phases (α, γ′, and γ) in the aged structure has also been desired.
Patent document 1: Japanese Patent Laid-Open No. 62595/2001
Patent document 2: Japanese Patent Laid-Open No. 18031/1988
Patent document 3: Japanese Patent Laid-Open No. 88431/2002
Non-patent document 1: Materia Japan (a bulletin of The Japan Institute of Metals), Vol. 22, No. 4, p. 323